Production of hydrogen can be considered eco-friendly only if it is produced from a noncarboneaceous feedstock using a renewable energy source. Hydrogen production by means of water splitting using solar energy is considered the “Holy Grail” of the hydrogen economy. Water splitting can be accomplished either directly in a single step or indirectly by multiple steps. Direct thermal decomposition of water is an energy intensive process that requires temperatures over 2500° C. The main obstacle to this approach is that hydrogen and oxygen (O2) evolved simultaneously in one reactor can readily recombine to form water—i.e. by back reaction. Combination of photovoltaic (PV) cells coupled to water electrolysis often serves as the benchmark solar hydrogen production process with which the performance of the other solar based hydrogen generation processes are appraised. PV cell efficiencies vary from 6% for amorphous silicon solar cells to more than 40% with multiple-junction research lab PV cells.
Solar cell energy conversion efficiencies for commercially available mono-crystalline silicon PV cells are around 14-16%. The highest efficiency PV cells such as multi junction cell based on gallium arsenide or indium selenide are still too expensive and not yet economical to use. On the other hand, water electrolysis is a well-developed technology with energy conversion efficiencies in the range of approximately 70-75%. Therefore the total solar to hydrogen efficiency of a PV-electrolysis system is in the range of 10-12%. The most commonly used PV cells only employ a portion of the solar energy (10 to 12%) while most of the solar thermal heat (88 to 90%) goes unutilized.
Unlike PV-electrolysis, water splitting processes based on the photocatalytic and photoelectrochemical methods provide a single step direct conversion of solar energy into the chemical energy of hydrogen. In the photocatalytic process, a photocatalyst converts the high-energy photons in solar spectrum into electron-hole pairs that promote redox reactions involving water to produce hydrogen and oxygen. In 1998, Khaselev and Turner reported that the hydrogen production efficiency of 12.4% for a monolithic photoelectrochemical-photovoltaic device based on the short-circuit current and the lower heating value of hydrogen as described in Khaselev O. and Turner J. A., “A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting,” Science, 280(17), pp. 425-7, 1998. The electrolyte used by Khaselev and Turner was 3 M sulfuric acid aqueous solution and the high cell output could only be maintained for less than 34 minutes after which the efficiency began to drop precipitously. According to Licht, S., Wang, B., Mukerji, S., Soga, T., Umeno, M. and Tributsch, H., “Over 18% Solar Energy Conversion for Generation of Hydrogen Fuel; Theory and Experiment for Efficient Solar Water Splitting,” Int. J. of Hydrogen Energy, 26, pp. 653-659, 2001, Licht and co-workers reported a solar to hydrogen energy conversion efficiency of more than 18% using a multi-junction photoelectrode in 1M HClO4 electrolyte and an artificial light source with Air Mass 0 (AM0) filter radiating with an intensity of 135 mW/cm2. However, no information was given with regard to the photo-electrode life in their work.
Thermodynamically, water splitting requires a minimum Gibbs free energy of 237.1 kJ per mole of water decomposed at 25° C. and 1 atm of pressure corresponding to 1.229 eV. Considering prevailing over potentials, water splitting requires upwards of 2.0 V or above. In a solar photocatalytic process, this implies the requirement for a wide band gap of greater than 2.0 eV photocatalyst. There are conflicting requirements for what makes a suitable photocatalyst in conducting water splitting reactions. In order to utilize the solar spectrum as broadly as possible, a semiconductor with narrow band gap energy is needed, however, electron-hole pairs generated by a low band gap semiconductor do not possess sufficient redox potential to engender water splitting that normally requires at least 2.0 eV of energy.
In case of a wide band gap semiconductor such as TiO2 (band gap energy of 3.0 eV) only a small portion of solar spectrum would be absorbed. Thus, for a semiconductor photocatalyst to be useful for water splitting, it must have several attributes as follows 1) its band gap must be wider than about 1.7 eV; 2) it must have a suitable minority band edge and Fermi level that cover both H2 and O2 evolution potentials; 3) it must be stable in very acidic or very alkaline solutions; 4) it must possess high efficiency for conversion of photons to electron-hole pairs; and 5) electron-hole pairs must be able to rapidly migrate to the semiconductor surface where redox reactions can readily occur thwarting charge recombination as described in Deutsch, T. G., Koval, C. A. and Turner J. A., “III-V Nitride Epilayers for Photoelectrochemical Water Splitting: GaPN and GaAsPN,” J. Phys. Chem. B, 110, pp. 25297-307, 2006. Moreover, these conditions have to be met simultaneously. To date, no such catalyst has been found.
Another approach to direct water splitting involves the use of sacrificial reagents—see, for example, the review by Liu and co-workers as described in Liu, H., Yuan, J., and Shangguan, W., “Photochemical Reduction and Oxidation of Water Including Sacrificial Reagents and Pt/TiO2 Catalyst,” Energy & Fuels, 20(6), pp. 2289-92, 2006. The central premise in this approach is that lower potentials would be necessary to evolve hydrogen if a sacrificial reagent is present as opposed to that required for direct unassisted water splitting. Therefore, the efficiency of H2 or O2 production from such systems can be significantly higher than direct water splitting. Several redox systems have been extensively investigated including electron donor systems such as: CH3OH or C2H5OH, Na2EDTA, Na2SO3, Na2S and NaI or KI; as well as the electron scavenger systems, e.g. AgNO3 and Fe(NO3)3. Although sacrificial reagent redox systems require lesser energy and can be carried out under milder conditions, a consumable reagent is required to produce hydrogen (or oxygen) from water. Only H2SO3—H2SO4 system can form a closed cycle if the problem with sulfur formation during H2SO3 oxidation can be satisfactorily addressed.
Thermochemical water splitting cycles (TCWSCs) utilize two or more chemical reactions (steps) that together form a closed loop with an overall reaction being the splitting of water and co-production of hydrogen and oxygen. Energy is typically input into one or more steps constituting a TCWSC. The basic concept behind the use of TCWSCs is to partition the total energy required for splitting water into several smaller and more manageable quantities input into the various steps within the cycle so that each step requires a portion of the total water splitting energy needed (ΔH°w,liquid=285.9 kJ/mol and ΔH°w, gas=241.83 kJ/mol at 25° C., 1 atm). FIG. 1 illustrates a three-step TCWSC in which the total energy (ΔHw) required for water splitting is segmented as follows:ΔHw=ΔH1+ΔH2+ΔH3  (1)Each step requires lesser amount of energy than the total water splitting energy as follows:ΔH1<ΔHw; ΔH2<ΔHw; ΔH3<ΔHw.  (2)
At least two steps are needed to form a TCWSC: H2 evolution step and O2 production step. If the energy required for one step of a TCWSC (typically, the oxygen evolving step) becomes greater than that needed to carry out direct water splitting, i.e. ΔHi>ΔHw, it implies that the cycle is a pseudo TCWSC. Since more energy than that needed for direct water splitting is typically stored in the products from oxygen generation step, hydrogen production step of the cycle can be considerably less energy intensive or even exothermic. Pseudo TCWSCs constitute a highly endothermic process for absorbing and storing the solar thermal heat at very high temperatures by use of solar cavity-receivers operating at temperatures above 2000° C. and mean solar flux concentration ratios, CR, exceeding 5000 as described in T-Raissi, A., Muradov, N., Huang, C. and Adebiyi, O., “Hydrogen from Solar via Light-Assisted High-Temperature Water Splitting Cycles,” J. Solar Energy Engineering, 129, pp. 184-9, 2007.
Unlike direct thermal decomposition of water that requires high temperature separation of O2 from H2, pseudo TCWSCs need to separate oxygen from an oxide (CO or MxOy) and a metal vapor to stop reverse reactions. Rapid quenching can help reduce the recombination rate of the products formed. FIG. 2 depicts the concept of pseudo TCWSCs having one step that consumes more energy than that needed for direct water splitting. There are three classes of pseudo TCWSCs: nonmetal oxide, metal/metal oxide and metal oxide/metal oxide cycles as described in Bilgen, E., Ducarroir, M., Foex, M., Sibieude, F., and Trombe, F., “Use of Solar Energy for Direct and Two-Step Water Decomposition Cycles,” Int. J. Hydrogen Energy, 2(3), pp. 251-7, 1977; Steinfeld, A., “Solar Hydrogen Production via Two-Step Water Splitting Thermochemical Cycle Based on Zn/ZnO Redox Reaction,” Int. J. Hydrogen Energy, 27, pp. 611-9, 2002; and Abanades, S., Charvin, P., Flamant, G., and Neveu, P., “Screening of Water-Splitting Thermochemical Cycles Potentially Attractive for Hydrogen Production by Concentrated Solar Energy”, Energy, 31, pp. 2805-22, 2006.
Nonmetal Oxide Cycles:CO2(g)=CO(g)+½O2(g), ΔH=283.0 kJ/mol, 1700° C.  (3)CO(g)+H2O(g)=H2(g)+CO2(g), ΔH=−41.2 kJ/mol, 700° C.  (4)SiO2→SiO(g)+½O2 2977° C.  (5)SiO(g)+H2O→SiO2+H2 2656° C.  (6)Reaction (3) requires higher energy than that needed for direct water thermolysis, ΔH°w, gas=241.83 kJ/mol.Metal/Metal Oxide TCWSCs:MxOy=xM+(y/2)O2, (endothermic), ΔH°>ΔH°W  (7)xM+yH2O=MxOy+yH2; (exothermic), ΔG<0  (8)
Some metal and metal oxide based pseudo TCWSCs described in Abanades et al. are given below:MoO2(s)→Mo+O2 3713° C.  (9)Mo+2H2O→MoO2(s)+2H2 1543° C.  (10)WO3(s)→W+3/2O2 3910° C.  (11)W+3H2O→WO3(s)+3H2 884° C.  (12)SnO2→Sn+O2 2650° C.  (13)Sn+2H2O→SnO2+2H2 600° C.  (14)ZnO→Zn+½O2 2000° C.  (15)Zn+H2O→ZnO+H2 100° C.  (16)Some low temperature metal/metal oxide cycles do not belong to pseudo TCWSCs described in Abanades et al. include:Hg(g)+H2O→HgO(s)+H2 360° C.  (17)HgO(s)→Hg(g)+½O2 600° C.  (18)Cd(s)→H2O→CdO(s)+H2, electrolytic, 25° C.  (19)CdO(s)→Cd(g)+½O2 1400° C.  (20)The above two cycles use heavy metals Hg and Cd and generally viewed as not environmentally friendly cycles.Metal Oxide/Metal Oxide TCWSCs:In2O3→In2O+O2 2200° C.  (21)In2O+2H2O→In2O3+2H2 800° C.  (22)Fe3O4(s)→3FeO(s)+½O2 2200° C.  (23)3FeO(s)+H2O→Fe3O4S(s)+H2 400° C.  (24)Ni0.5Mn0.5Fe2O4 Ni0.5Mn0.5Fe2O4-x+(x/2) O2 1100° C.  (25)Ni0.5Mn0.5Fe2O4-xxH2O→Ni0.5Mn0.5Fe2O4+xH2 600° C.  (26)MnFe2O4+3CaO+(1-x)H2O→Ca3(Fe, Mn)3O8-x+(1-x)H2 1000° C.  (27)Ca3(Fe, Mn)3O8-x→MnFe2O4+3CaO+½(1-x)O2 600° C.  (28)
The overall thermal efficiency (ηoverall) (or 1st law efficiency) of a TCWSC is defined as the ratio of hydrogen chemical energy to total energy consumed by the cycle.
                              η          overall                =                                            n              ·              Δ                        ⁢                                                  ⁢                          H              f              °                                            Δ            ⁢                                                  ⁢                          H              total                                                          (        29        )            Where n is the total mole of hydrogen generated by the cycle, ΔH°f is enthalpy of water formation and ΔHtotal is the total energy input to the cycle to produce n moles of hydrogen. If the enthalpy formation of water in liquid state is used (at 298 K, ΔHf=−68.32 kcal/mol=285.9 kJ/mol), the efficiency thus calculated is called high heating value (HHV) efficiency, η(HHV). Some argue that the latent heat of condensation cannot be economically recovered and prefer using the low heating value (LHV) efficiency η(LHV) in which ΔH°f is the enthalpy of formation of water vapor at 298 K (ΔH°f=−57.41 kcal/mol=240.2 kJ/mol). The ratio η(HHV)/(LHV)=68.32/54.74=1.19. The figure of merit or Carnot efficiency (also, work or 2nd law efficiency) is defined as:
                              η          ⁡                      (            w            )                          =                                                            n                ·                Δ                            ⁢                                                          ⁢                              G                f                °                                                    Δ              ⁢                                                          ⁢                              H                total                                              =                                    237.2              ·              n                                      Δ              ⁢                                                          ⁢                              H                total                                                                        (        30        )            Where, ΔG°f is the Gibbs free energy of water formation (237.2 kJ/mol). Since early 1970s, when the concept of TCWSCs was introduced, much effort has been devoted to defining their efficiencies.
However, due to the fact that TCWSCs often contain several reaction steps as well as processes for the material transport and separation, precise determination of the efficiencies has been difficult. Inventors, Huang and Raissi have shown that efficiency determination for a TCWSC must be calculated based on the chemical process simulation in which a detailed flow sheet that takes into account for the material and energy balance as well as precise values of the chemical and physical properties of reactants and products as described in Huang, C., and T-Raissi, A., “A Perspective on Thermodynamics and Thermal Efficiency Calculations for Hydrogen Production via Thermochemical Water Splitting Cycles,” manuscript t to be submitted for publication.
FIG. 3 shows a simple flow diagram for a model TCWSC. Water is fed into the cycle and hydrogen and oxygen are the only output of the cycle. The hydrogen and oxygen production steps are connected by process steps involving chemical separation and recycling thus forming a closed cycle with an overall reaction of water decomposition into H2 and O2. Total energy needed to perform water splitting consists of four parts: energies required to generate H2 and O2 (i.e. ΔH1 and ΔH2), separation of reactant from products (ΔHS) and recycling of the reactants (ΔE). Then,ΔHTotal=ΔH1+ΔH2+ΔHS+ΔE  (30)
The efficiency of a TCWSPC depends upon if energy input to the cycle is maximally used for carrying out chemical reactions at same time minimizing the energy losses. There are a number of types of energy losses for TCWSPCs leading to a wide range of efficiency loss. In order to develop an innovative TCWSPC, it is important to analyze and evaluate the existing TCWSPCs in terms of energy losses. The energy losses in a TCWSPC can be separated into four major categories as follows:
1. Kinetic energy loss: this includes reaction activation energy, mass transportation energy and energy to overcome reverse reactions.
2. Heating energy loss: 100% heat recovery can not be achieved. Low temperature heating energy is not recoverable.
3. Separation energy loss: energy required to separate one product from another or from reactants is not recoverable. Separation of gas from gas or liquid from liquid is an energy intensive process. While separating a solid from another solid is extremely difficult or almost impossible.4. Transport energy loss which is electrical energy used to pump and move species.
It should be pointed out that heating energy can be recovered. From a viewpoint of chemical reactions, the efficiency of a TCWSPC can also be expressed as:η=(ΔHreaction)/ΔHtotal×100=(ΔHtotal−ΔHkinetics−ΔEtransportation−ΔHheat loss−ΔHseparation)/ΔHtotal×100
Transportation energy requirements for a TCWSPC consists a small portion of the total energy needed. On the other hand, heating energy loss can be minimized using a heat exchanger network. Pinch analysis can provide an optimization technique for heat exchange network with which heating loss can be minimized. Therefore, in most TCWSPCs, kinetic energy and separation energy represent major portions of the energy loess. If chemical reactions in a TCWSPC can be approximately reach their thermodynamic equilibria, the kinetic energy loss can be neglected. However, the major issues for most high temperature reactions are reverse reactions that require a quenching step to cool temperature rapidly to avoid this reaction, for example:CO2(g)=CO(g)+0.5O2(g)ZnO(s)=Zn(g)+0.5O2(g)SnO2→Sn+O2 
Apparently, separating one gas from a gas mixture and a liquid from a solution is not only an energy intensive process it also can cause material losses. For example, separation of SO2 from O2 requires a compression process to liquefy SO2 so that O2 can be separated. The O2 separated in this method can contain small amounts of SO2, indicating the loss of sulfur component. To separate H2 from CO2 in CO2—CO cycle (Reaction (4)) may need a Pressure Swing Adsorption process that can cause hydrogen loss. Separating H2O from H2SO4 is another typical example of liquid and liquid separation that involves in all the sulfuric acid decomposition based TCWSPCs. The separation represents a major energy loss and determines the efficiency of the entire cycle. Some TCWSPCs comprise a step of separating a solid from a solid mixture indicating that the operation is complicated and the cycle efficiency is low. Based upon the energy loss analyses, the cycle efficiency can be estimated as:η=(ΔHreaction)/ΔHtotal×100 (ΔHtotal−ΔHkinetic−ΔHseparation)/ΔHtotal×100
As reported in the literature, since the decomposition of H2SO4 and MSO4 at higher temperatures can reach thermodynamic equilibria, the kinetic energy loss can be neglected, therefore, the efficiency of H2SO4 and MSO4 based TCWSPCs can be simplified as:η=(ΔHreaction)/ΔHtotal×100≈(ΔHtotal−ΔHseparation)/ΔHtotal×100This partially shows why H2SO4 and MSO4 based TCWSPCs are high efficiency processes and therefore are widely studied.
The oxygen producing step in the sulfur family cycles is the decomposition of sulfuric acid or a metal sulfate. The energy required for generating hydrogen and oxygen from water is immense (286 kJ/mol). However, the energy input for the decomposition of H2SO4, calculated using Thermfact and GTT-Technologies FactSage™ 5.5 thermochemical software, is only about 80.9% of the total energy required for water splitting (i.e. 286 kJ/mol):
H2SO4 = H2O + SO3ΔH° 298K = 87.1 kJ/molSO3 = SO2 + ½O2ΔH° 298K = 144.2 kJ/molfor which the overall reaction is:
H2SO4 = H2O + SO2 + ½O2ΔH° 298K = 231.3 kJ/mol
The remaining 19.1% of the energy required to split water is then used for the H2 production step. Note that about 80.9% of the total solar irradiance comprising of mostly thermal energy with wavelengths above 520 nm can be utilized for the decomposition of sulfuric acid in the oxygen generation step of the sulfur-family cycles. The remaining 19.1% of the total solar irradiance which is photonic energy at wavelengths less than about 520 nm will be used for the hydrogen production step of the cycle. In other words, for optimum overall cycle efficiency, it is necessary that the oxygen production step to utilize 80.9% of the solar irradiance as mostly thermal radiation above a wavelength of approximately 520 nm and the hydrogen generation step to consume the remaining 19.1% of solar power, at wavelengths shorter than 520 nm—preferably, within a photolytic and/or photocatalytic reactor.
Large-scale solar concentrators typically utilize parabolic reflectors in the form of trough, tower, or dish systems. These solar concentrators are characterized in terms of their mean flux concentration ratio CR over an area Sa at the receiving focal plane as follows:CR=qs/I where qs (W/m2) refers to the solar flux intercepted by unit area of the receiver at the focal plane and I (W/m2) is the incident normal beam isolation. CR is often expressed in units of “suns” when normalized to I=1000 W/m2 as described in Steinfeld, A., “Solar Thermochemical Production of Hydrogen—A Review,” Solar Energy, 78, pp. 603-15, 2005. The solar flux concentration ratio typically obtained is at the level of 100, 1000, and 10,000 suns for trough, tower, and dish systems, respectively. The most suitable concentrators for applications involving solar thermochemical water splitting cycles include tower and dish systems.
Due to the high 1st and 2nd law efficiencies of sulfuric acid based cycles, to date, more than 20 sulfuric acid and/or metal sulfate decomposition based TCWSCs have been reported in the literature. Despite difficulties that challenge efficient electrolytic oxidation of sulfur dioxide (SO2), the Westinghouse hybrid cycle still remains as one of the most studied TCWSCs conceived for the production of hydrogen from water. The Westinghouse cycle described in Brecher, L. E., Spewock, S., et al., “Westinghouse Sulfur Cycle for the Thermochemical Decomposition of Water,” Proceedings of the 1st World Hydrogen Energy Conf., 1 9A, 1-16, 1976 is as follows:SO2(g)+2H2O(l)=H2+H2SO4(aq) 77° C. (electrolysis)  (31)H2SO4(g)=SO2(g)+H2O+½O2 850° C. (thermolysis)  (32)
The many advantages of the Westinghouse cycle have been widely reported and discussed in the literature. Westinghouse cycle is known to be hampered by the low solubility of SO2 in water and challenges presented by the acidity of the SO2 electrolytic oxidation process. To date, many efforts have been made to improve the efficiency of the electrolytic process for oxidation of SO2. Past activities have involved the use of a depolarized electrolyzer as well as addition of a third process step—examples include S—I, S—Br and S—Fe cycles given below:
Ispra Mark 13 Sulfur-Bromine Cycle:Br2(l)+SO2(g)+2H2O(l)→2HBr(aq)+H2SO4(aq) 77° C.  (33)H2SO4(g)→SO2(g)+H2O(g)+½O2(g) 850° C.  (34)2HBr(aq)→Br2(aq)+H2(g) (electrolysis) 77° C.  (35)General Atomics' Sulfur-Iodine Cycle:I2+SO2(g)+2H2O(l)→2HI(a)+H2SO4(aq) 100° C.  (36)H2SO4(g)→SO2(g)+H2O(g)+½O2(g) 850° C.  (37)2HI→I2(g)+H2(g) 450° C.  (38)Sulfur-Iron Cycle:Fe2(SO4)3(aq)+SO2+2H2O→2FeSO4(aq)+2H2SO4 25° C.  (39)H2SO4(l)→SO2(g)+H2O(g)+½O2(g) 850° C.  (40)2FeSO4(aq)+H2SO4(aq)→Fe2(SO4)3(aq)+H2(g) 25° C.  (41)
Although these cycles address some of the challenges associated with water splitting, especially with regard to the solubility of SO2 in water, they have issues of their own. For example, efficient separation of sulfuric acid from reaction products such as HI, HBr or FeSO4 presents a challenge. Additionally, the pH of the solutions remains problematic. In fact, this problem becomes more acute due to the generation of other acids such as HI and HBr.
The second approach is to introduce a metal oxide as a catalyst to convert low concentration sulfuric acid to metal sulfate that can be decomposed for the production of oxygen and sulfur dioxide, metal oxide. Sulfur dioxide and water are send back to sulfur decomposition based TCWSPCs for the production of hydrogen and sulfuric acid to close a cycle. Introducing ZnO into the Westinghouse TCWSPC a new modified ZnSO4 decomposition based Westinghouse cycle can be written as:SO2(g)+2H2O(l)=H2+H2SO4(aq) 77° C. (electrolysis)  (66)H2SO4(aq, 50 wt %)+ZnO(s)=ZnSO4.H2O(s) 80˜350° C.  (67)ZnSO4.H2O(s)=ZnSO4(s)+H2O(g) 450° C.  (68)ZnSO4(s)=SO2(g)+½O2+ZnO(s) 850° C.  (69)
Similarly, by adding metal oxide catalysts to the Ispra Mark 13 sulfur-bromine cycle, General Atomics' sulfur-iodine cycle and sulfur-iron cycle, a number of new, modified metal sulfate based TCWSCs can be devised. However, if solar energy is use to drive these cycles, only solar thermal energy can be utilized resulting in degrading the photonic portion of solar spectrum to lower grade heat. Secondly, although metal sulfate cycles have overcome some difficulties associated with the sulfuric acid based TCWSPCs, they have some other issues that need to be addressed. A fundamental difficulty of this type of TCWSPCs is that the hydrogen production step:MO(s)+H2O+SO2(g)→MSO4(s)+H2(g)is thermodynamically unfavorable. Besides that, the separation of H2 from unreacted SO2(g) is an energy intensive process.
For solar driven water splitting, Abanades and co-workers screened 280 TCWSCs as disclosed in Abanades, S., Charvin, P., Flamant, G., and Neveu, P., “Screening of Water-Splitting Thermochemical Cycles Potentially Attractive for Hydrogen Production by Concentrated Solar Energy”, Energy, 31, 2805-22, 2006. They selected 30 TCWSCs as promising solar driven cycles for further investigation. Among these cycles, there were nine metal sulfate based TCWSCs—almost ⅓ of all selected cycles. This implies that the decomposition of H2SO4 or MSO4 presents an effective method for heat absorbing step of the TCWSCs. The General Atomics' S—I cycle was not among the selected candidates considered suitable for solar interface by Abanades et al due to the difficulties in separating HI from water. Three examples of metal sulfate cycles are given below:MnSO4→MnO+SO2+½O2 1100° C.  (42)MnO+H2O+SO2→MnSO4+H2 250° C.  (43)FeSO4→FeO+SO2+½O2 1100° C.  (44)FeO+H2O+SO2→FeSO4+H2 250° C.  (45)CoSO4→CoO+SO2+½O2 1100° C.  (46)CoO+H2O+SO2→CoSO4+H2 250° C.  (47)3FeO(s)+H2O→Fe3O4(s)+H2 200° C.  (48)Fe3O4(s)+FeSO4→3Fe2O3(s)+3SO2(g)+½O2 800° C.  (49)3Fe2O3(s)+3SO2(g)→3FeSO4+3FeO(s) 1800° C.  (50)3FeO(s)+H2O→Fe3O4(s)+H2 200° C.  (51)Fe3O4(s)+3SO3→3FeSO4(g)+½O2 800° C.  (52)FeSO4→3FeO(s)+SO3 2300° C.  (53)Fe2O3(s)+2SO2(g)+H2O→2FeSO4(s)+H2 200° C.  (54)2FeSO4(s)→3Fe2O3(s)+SO2(g)+SO3(g) 700° C.  (55)SO3(g)→SO2(g)+½O2 2300° C.  (56)6Cu(s)+3H2O→3Cu2O(s)+3H2 500° C.  (57)Cu2O(s)+2SO2(g)+1.5O2→2CuSO4 300° C.  (58)2Cu2O(s)+2CuSO4→6Cu(s)+2SO2+3O2 1750° C.  (59)Cu2O(s)+H2O(g)→Cu(s)+Cu(OH)2 1500° C.  (60)Cu(OH)2+SO2(g)→CuSO4+H2 100° C.  (61)CuSO4+Cu(s)→Cu2O(s)+SO2+½O2 1500° C.  (62)SO2+H2O+BaMoO4→BaSO3+MoO3+H2O 300° C.  (63)BaSO3+H2O→BaSO4+H2  (64)BaSO4(s)+MoO3(s)→BaMoO4(s)+SO2(g)+½O2 1300° C.  (65)
The methods and systems of the present invention provide a class of new thermochemical water splitting cycles that utilize solar photonic energy for the production of hydrogen and solar thermal heat for oxygen production.